The Red Devils took the lead through Henrikh Mkhitaryan in the 36th minute before wasting a host of golden opportunities to kill the quarter-final first leg tie.
United's profligacy in front of goal proved to be costly as they conceded to Leander Dendoncker on 86 minutes which gives them only a slender advantage with the away goal for next week's return bout at Old Trafford.
He told BT Sport: "In my poor English I cannot find a better word [than sloppy]. You have to play more seriously.
"Put the performance of two or three of our attacking players together and you squeeze not much juice out of it. Rashford, Lingard, Ibrahimovic, Martial. They were very similar.
"But the people behind very solid very focused. The defenders did the serious work but the people who had to kill the game didn't.
"We start the game at Old Trafford in front, but we had all the conditions to kill the game."